Most driving motors used in electrical devices such as air conditioners, water heaters, air cleaners, copying machines, and printers are brushless DC motors because they have advantages such as long life, high reliability, and ease of speed control.
When a speed-controllable motor running at a constant speed is decelerated, the motor begins to act as an electric generator. More specifically, the so-called regenerative phenomenon occurs where the motor supplies power to the power supply or the drive circuit, which are supposed to supply power to the motor.
FIG. 10 is a configuration diagram including a conventional motor driving device for driving a brushless DC motor as described above. FIG. 11 is a diagram showing the operation of this motor driving device. FIG. 12 is a diagram showing the regenerative phenomenon observed in the motor driving device. FIG. 13 is a diagram showing a voltage increase in a DC power supply caused by the regenerative phenomenon in the motor driving device.
The following is a description, with reference to drawings, of the structure and operation of the conventional motor driving device for driving a brushless DC motor using sinusoidally pulse-width-modulated drive signals, and of a regenerative phenomenon observed in the conventional technique.
FIG. 10 shows motor driving device 800, which is supplied with DC power from DC power supply 805. Motor driving device 800 converts the DC power to driving power, and supplies the driving power to motor 810, which is a brushless DC motor. Motor driving device 800 receives a speed command signal Sref and a switching signal HL as command information from host device 806, and also receives a position detection signal CS and a speed detection signal N from motor 810.
Motor 810 includes U-phase drive winding 811, V-phase drive winding 813, and W-phase drive winding 815, which are supplied with the driving power from motor driving device 800.
Motor driving device 800 includes inverter 820, inverter drive unit 830, and speed control unit 840. Speed control unit 840 generates a drive control signal VSP for controlling the speed of motor 810 based on the command information and the speed detection signal N, and sends it to inverter drive unit 830. Inverter drive unit 830 generates drive signals for drive-controlling inverter 820 based on the drive control signal VSP, and then drives inverter 820. Inverter 820 converts the supplied DC power into driving voltages according to the position detection signal CS and the drive control signal VSP, and supplies the driving voltages to motor 810.
Inverter 820 includes positive-electrode-side switch elements 821, 823, and 825 for connecting drive windings 811, 813, and 815 of motor 810 to a positive-electrode-side power supply line Vp and negative-electrode-side switch elements 822, 824, and 826 for connecting these drive windings to a negative-electrode-side power supply line Vn.
Inverter drive unit 830 includes waveform generation unit 831 and pulse-width modulation unit 832. Waveform generation unit 831 generates a sinusoidal waveform signal WF according to the position detection signal CS received from motor 810. Pulse width modulation unit 832 generates drive signals UH, VH, WH, UL, VL, and WL, which have been subjected to pulse width modulation (hereinafter sometimes referred to as “PWM”) according to the waveform signal WF.
The drive signals UH, VH, and WH have a phase difference of an electrical angle of 120 degrees from each other, and the drive signals UL, VL, and WL also have a phase difference of an electrical angle of 120 degrees from each other. These drive signals are connected to the switch elements (hereinafter sometimes referred to simply as “switches”) of inverter 820 so as to turn them on and off as shown in FIG. 10.
The following is a description of the operation of the motor driving device for driving the above-described conventional brushless DC motor. The following description will be focused on the action associated with U-phase drive winding (hereinafter sometimes referred to simply as “winding”) 811, which is connected to the output U of inverter 820.
First, host device 806 sends the speed command signal Sref and the switching signal HL as the command information to speed control unit 840. Host device 806 is composed of a microcomputer, a DSP, or the like. The speed command signal Sref is a signal for commanding the speed of motor 810. The switching signal HL is a control signal for changing the control gain according to the speed setting of motor 810 indicated by the speed command signal Sref.
In order to control motor 810 to drive at the speed indicated by the speed command signal Sref, speed control unit 840 adjusts the drive control signal VSP so as to equalize the speed command signal Sref with the speed detection signal N received from motor 810. Waveform generation unit 831 generates a sinusoidal waveform signal WF whose amplitude corresponds to the drive control signal VSP.
FIG. 11 shows the sinusoidal waveform signal WF thus generated by waveform generation unit 831, and a triangular waveform signal CY, which is a PWM carrier signal generated within pulse-width modulation unit 832. Pulse width modulation unit 832 compares the waveform signal WF with the carrier signal CY. Switches 821 and 822 of inverter 820 are tuned on and off complementarily according to the comparison result. This allows the driving voltage U shown in FIG. 11 to be outputted from inverter 820 and applied to drive winding 811. As a result, drive winding 811 is supplied with a U-phase drive current Iu and generates an induced voltage Uemf. From the instantaneous viewpoint, the driving voltage U is a pulse-like voltage alternately changing between the positive-electrode-side voltage and the negative-electrode-side voltage of DC power supply 805. When the mean value is calculated based on the pulse width modulation principle, on the other hand, the driving voltage U is a sinusoidal voltage corresponding to the waveform signal WF. As a result, drive winding 811 is supplied with the same sinusoidal voltage as in the U-phase waveform signal WF. The term “complementarily” means that while one switch is in the ON state, the other is in the OFF state, and while the one switch is in the OFF state, the other is in the ON state.
FIG. 11 also shows the detailed timings at which the positive-electrode-side switches and the negative-electrode-side switches are turned on and off complementarily. The drive signal UH turns on and off switch 821, and the drive signal UL turns on and off switch 822. Switch 821 is in the ON and OFF states when the drive signal UH is at the levels H and L, respectively. Switch 822 is in the ON and OFF states when the drive signal UL is at the levels H and L, respectively. Specifically, when the ON and OFF states of these switches are changed over, there is provided a brief moment, like a time “td” shown in FIG. 11. The time “td” is called a dead time or an on delay, during which period both switches are in the OFF state. This period is provided as a well-known technique to prevent a short circuit of DC power supply 805.
V-phase drive winding 813 and W-phase drive winding 815 are supplied with sinusoidal voltages as a driving voltage V and a driving voltage W, respectively, from inverter 820 in the same manner as U-phase drive winding 811, while keeping the phase difference of an electrical angle of 120 degrees between the U-, V-, and W-phases.
Thus, the sinusoidal voltages having an amplitude (peak value) corresponding to the drive control signal VSP are applied to drive windings 811, 813, and 815 different in phase from each other. As a result, motor 810 is sine-wave driven while the driving power to the windings is adjusted to control the speed.
The following is a description of the regenerative phenomenon occurring in motor driving device 800.
In FIG. 12, the waveform signal WF corresponds to the driving voltage U outputted from inverter 820. The waveform signal WF shows the operation of motor 810 when the mean value of the driving voltage U (corresponding to the waveform signal WF) becomes smaller than the induced voltage Uemf generated in drive winding 811. The driving voltage can become smaller than the induced voltage when the peak value of the waveform signal WF is reduced, for example, in order to decelerate the motor.
First, in a period “a” of FIG. 12, switch 821 is in the ON state, and switch 822 is in the OFF state. As a result, drive winding 811 is connected to the positive-electrode-side power supply line Vp of DC power supply 805, and the instantaneous value of the driving voltage U becomes the voltage of the positive-electrode-side power supply line Vp. In the period “a”, the driving voltage U is higher than the induced voltage Uemf, thus increasing the current Iu of drive winding 811. The increment depends on the voltage (shown in the area hatched in the period “a”) obtained by subtracting the induced voltage Uemf from the driving voltage U. When the mean value of the driving voltage U is smaller than the induced voltage Uemf, however, the difference is small, and the current increase is also small.
Next, in a period “b”, switch 821 is in the OFF state, and switch 822 is in the ON state. As a result, drive winding 811 is connected to the negative-electrode-side power supply line Vn of DC power supply 805, and the instantaneous value of the driving voltage U becomes the voltage of the negative-electrode-side power supply line Vn. In the period “b”, the driving voltage U is lower than the induced voltage Uemf, thus decreasing the current Iu of drive winding 811. The decrement depends on the voltage (shown in the area hatched in the period “b”) obtained by subtracting the driving voltage U from the induced voltage Uemf. When the mean value of the driving voltage U is smaller than the induced voltage Uemf, the difference is large, and the current decrease is also large.
In a period “b1” of the period “b”, the current Iu reaches drive winding 811 after flowing through switch 822 or the diode connected antiparallel thereto, and then continues to decrease. In a period “b2” before which the current Iu decreases to zero, the direction of the current is inverted. As a result, the current Iu begins to flow to the negative-electrode-side power supply line Vn from drive winding 811 via switch 822. The current Iu at this moment is supplied from the induced voltage Uemf, which is opposite to the direction in which the current for driving the motor is supposed to flow.
In a period “c”, switch 821 is in the ON state, and switch 822 is in the OFF state as in the period “a”. As a result, the current Iu increases as in the period “a”, but the increment is too small to reverse the current direction so as to return to the original direction. In the period “c”, the current Iu flows to the positive-electrode-side power supply line Vp from drive winding 811 via switch 821 or the diode connected antiparallel thereto. The current Iu is supplied from the induced voltage Uemf, which is opposite to the direction in which the current for driving the motor is supposed to flow as in the period “b2”.
Next, in a period “d”, switch 821 is in the OFF state and switch 822 is in the ON state as in the period “b”. As a result, the current Iu continues to greatly decrease as in the period “b”. In the period “d”, the current Iu flows to the negative-electrode-side power supply line Vn from drive winding 811 via switch 822. As a result, a larger current Iu than in the period “b2” is supplied from the induced voltage Uemf.
Driving the motor requires supplying the current Iu to the induced voltage Uemf of drive winding 811. However, as described above, there occurs a phenomenon that the current Iu is, on the contrary, supplied from the induced voltage Uemf. When this phenomenon continues, every time switch 821 is turned on and switch 822 is turned off, the current Iu flows to the positive-electrode-side power supply line Vp from drive winding 811 via switch 821 or the diode connected antiparallel thereto. This phenomenon occurs also in windings 813 and 815, and the currents flown from these windings flow to the positive-electrode-side electrode of DC power supply 805 via the positive-electrode-side power supply line Vp.
This results in the so-called regeneration where the motor acts as an electric generator and supplies power to the DC power supply, which is supposed to supply power to the motor.
FIG. 13 is a diagram showing an increase in an output voltage VDC of DC power supply 805 caused by the above-described regenerative phenomenon.
When the speed command signal Sref is decreased as shown by the broken line in FIG. 13, that is, a deceleration command is issued, the motor is decelerated. At this moment, speed control unit 840 decreases the drive control signal VSP according to the difference between the speed command signal Sref and the speed detection signal N. The decrease in the drive control signal VSP results in a decrease in the output voltage of inverter 820, that is, the driving voltage of each of drive windings 811, 813, and 815. The driving voltages applied to these windings are lower than the induced voltages generated in the windings. When the driving voltages applied to the windings are thus lower than the induced voltages, the above-described regenerative phenomenon occurs, thereby increasing the output voltage VDC of DC power supply 805. The comparative size between the driving voltage applied to a winding and the induced voltage corresponds to the comparative size between the drive control signal VSP and the speed detection signal N shown in FIG. 13.
As described hereinbefore, motors including brushless DC motors generally cause such regenerative phenomena. Therefore, there has been proposed to return the power generated by a regenerative phenomenon to the power supply side for its effective use or to protect the drive circuit or the power supply circuit from overvoltage due to the regenerative phenomena (see, for example, Patent Literature 1).
FIG. 14 is a configuration diagram of a conventional regeneration control device. This device returns a regeneration current caused by a regenerative phenomenon to the power supply circuit, and reduces the increase in the DC circuit voltage due to the regenerative phenomenon during the period which is switched to the power-supply regeneration. The conventional regeneration control device thus protects the inverter device from DC circuit overvoltage during regeneration.
In FIG. 14, thyristor converter 92 acts as a converter during power running so as to convert the AC voltage of AC power supply 91 to a DC voltage, and also acts as an inverter during regeneration operation. Capacitor 93 is connected in parallel on the DC side of thyristor converter 92. Inverter circuit 94 converts the DC power received from thyristor converter 92 to AC power so as to variable-speed-control induction motor 95. Speed control circuit 914 calculates a current command Iref for nullifying the deviation between a motor speed Nfbk detected by speed detector 98 and a speed command Nref. Control means 920 controls the output voltage and frequency of inverter circuit 94 based on the current command Iref.
When induction motor 95 is decelerated and switched from power running to regeneration, regeneration determiner 912 determines the occurrence of a switch-over based on the speed command Nref and the current command Iref. Current-change-rate limiting circuit 921 is provided between speed control circuit 914 and control means 920. Current-change-rate limiting circuit 921 reduces the time change rate of the current command Iref only during the period while thyristor converter 92 is switched to regeneration conversion according to the determination output of regeneration determiner 912.
In the conventional regeneration control device thus structured, when decelerated by reducing the speed command Nref, induction motor 95 is switched from operating as a motor to operating as an electric generator. In other words, the energy of the mechanical system is transferred to capacitor 93 through inverter circuit 94 and regenerated as electrical energy. Therefore, this conventional regeneration control device first pays attention to the time change rate of the absolute value of the speed command Nref. Regeneration determiner 912 determines whether the time change rate is negative, or on the increase after the current command Iref, which is the output of speed control circuit 914 is switched from power running to regeneration. Then, delay circuit 913 reduces the rate of increase in the current command by reducing the change rate of the regeneration current of current-change-rate limiting circuit 921 during the time until thyristor converter 92 has been switched from a converter to an inverter and starts regeneration. In this manner, the conventional regeneration control device reduces the charging current of capacitor 93, and hence, the increase in the charge voltage. When thyristor converter 92 has been switched to regeneration, the DC circuit voltage is reduced to a predetermined value by voltage control circuit 931.
Thus, the conventional regeneration control device provides a means to project the inverter device from DC circuit overvoltage during regeneration instead of providing a regenerative discharge resistor for consuming regenerative energy or increasing the capacitor capacity for storing regenerative energy.
The conventional regeneration control device allows efficient use of electric power by returning electric power generated by a regenerative phenomenon to the power supply side. On the other hand, however, the device has problems that the motor is inconvenient to use and that a larger number of peripheral circuits are required, thus leading to a cost increase. For example, the regeneration control device requires a circuit component for regeneration operation such as the thyristor converter capable of being switched between a converter and an inverter, or a power supply capable of absorbing regenerative power. Therefore, in terms of convenience and cost, it is desirable for a host device including a motor such as an air conditioner or a copying machine desires to have a motor device which can be operated only by connecting the motor to a power supply device capable of supplying a rated voltage and current without the need to consider regeneration.
Patent Literature 1: Japanese Patent Unexamined Publication No. 2007-215282